The present invention relates to electric lamps and, more specifically, to fluorescent electrodeless lamps operated at low and intermediate pressures without the use of ferrite at frequencies from 20 KHz to 200 MHz.
Electrodeless fluorescent lamps utilizing an inductively coupled plasma were found to have high efficacy and lives that are longer than that of conventional fluorescent lamps employing hot cathodes.
The plasma that generates UV and visible light in a lamp is produced in a glass (or quartz) envelope filled with an inert gas such as argon or krypton and a metal vapor such as mercury, sodium or cadmium. To generate the plasma, an induction coil is positioned in close proximity to the lamp envelope.
The prior art teaches two types of electrodeless fluorescent lamps, ones with a ferrite core and ones without the ferrite core. The lamps operate at a frequency of 2.65 MHz that is allowed in many countries and employ an induction coil wrapped around a ferrite core (U.S. Pat. No. 4,568,859 by Houkes et al., U.S. Pat. No. 5,343,126 by Farral et al.). The use of the ferrite core is necessary to operate at low coil current and hence at low coil/core power losses.
However, the ferrite core gets hot during operation especially at high power, P greater than 50 W. Therefore, cooling of the ferrite core is required to maintain its temperature below Curie point (U.S. Pat. No. 5,006,752 by Eggink et al., U.S. Pat. No. 5,572,083 by Antonis et al.) The longer a lamp bulb, the longer and heavier the ferrite core must be. This makes the ferrite core cooling structure very large, heavy, and cumbersome.
It is obvious that an electrodeless fluorescent lamp without ferrite core would be simpler in construction and less expensive.
In the prior art, U.S. Pat. No. 5,013,975 by Ukegawa et al., shows an induction coil wrapped around a tube. However, patentees did not disclose either the shape or size of the tube (eg. a sphere, cylinder, linear tube). The induction coil structure and its location were not specified either. U.S. Pat. No. 5,013,975 does not present the data about RF power, frequency range, lumen output, light spacial distribution as well as about lamp power efficiency and efficacy. The claims are supported with a few figures where the tube looks like a sphere wrapped with an induction coil near the sphere""s diameter. This approach seems to provide reasonable plasma uniformity in bulbs of spherical shape or in bulbs of short length but it does not generate an axially uniform plasma in a long linear tube with the length much larger than diameter.
In U.S. patent application Ser. No. 09/256,137 by Popov and assigned to a common assignee, an electrodeless fluorescent lamp is disclosed which operated without a ferrite core at frequencies from 150 KHz to 15 MHz and RF power from 50 W to 220 W. The lamp envelope had so-called xe2x80x9cclosed-loopxe2x80x9d shape that was employed in U.S. Pat. No. 3,500,118 by Anderson, and in U.S. Pat. No. 5,834,905 by Godyak et al. Both patents used ferrite cores.
The induction coil in the lamp described in U.S. patent application Ser. No. 09/256,137 has several windings (turns) that were positioned on the xe2x80x9catmosphericxe2x80x9d side of the envelope tube. The coil was located either on the xe2x80x9cinnerxe2x80x9d side of the closed-loop envelope or on the xe2x80x9couterxe2x80x9d side of the envelope. The envelope tube length was much larger than the tube diameter. The lamp power efficiency, Ppl/Plamp, and lamp efficacy, LPW, at f=260 KHz and RF power of 150 W were close to those of the lamp described in U.S. Pat. No. 5,834,905 that employed two ferrite cores and had the same tube shape and size.
Electrodeless lamps described in U.S. Pat. No. 5,834,905 and U.S. patent application Ser. No. 09/256,137 have an envelope of closed-loop (xe2x80x9ctokamakxe2x80x9d) shape. Such a shape provides the continuity of the discharge current inside the envelope. That is, the current forms a closed-loop path along the tube envelope walls. To make such a loop, the U.S. Pat. No. 5,834,905 teaches that the envelope has to be made from two straight glass tubes that are sealed to each other with the two short tubes. The other implementation of the closed-loop approach was to bend a liner tube in a circle and to connect both tube ends thereby forming the closed-loop path for the gas and discharge current within the envelope.
In order to provide conditions for a closed-loop current of an inductive discharge excited in a single straight tube, Kobayashi et al. (U.S. Pat. No. 4,864,194) divided the tube volume in two parts by the introduction of the partition along the tube axis. The induction coil was wrapped along the tube walls in its axial direction. However, the division of the tube volume in two parts results in the reduction of the tube""s effective radius that causes the increase of the discharge electric field and hence, power losses in the coil. Also, the introduction of the additional part (e.g. glass) in the lamp volume makes the lamp manufacturing process more complex and expensive.
We discovered a closed-loop path for the discharge current could be achieved inside of the linear single glass/quartz tube without the partition of the tube volume. The induction coil consists of a few windings (turns) wrapped along the tube walls in axial direction, normal to the diameter and parallel to the axis of the tube. The inductive discharge is generated along the tube walls with the current flowing along the walls in the tube axial direction thereby forming a closed-loop path inside the tube. As a result, the axially uniform plasma is generated along the whole length of the linear tube.
According to the present invention a novel approach is disclosed that results in an efficient ferrite-free electrodeless lamp that is operated at frequency from 100 KHz to 100 MHz. The lamp power efficiency and efficacy were found to be comparable to those of electrodeless closed-loop lamps described in U.S. Pat. No. 5,834,905 (Godyak et al.) and in our U.S. patent application Ser. No. 09/256,137.
The present invention comprises an electrodeless fluorescent lamp having a glass or quartz envelope made from linear single tube of any configuration, cross section and size. A filling of inert gas and vaporous metal such as mercury, cadmium, sodium is placed in the envelope. The metal vapor pressure is maintained below 1 Torr and the inert gas pressure is below 10 Torr. A protective coating is disposed on the inner surface of the envelope walls and a phosphor coating is disposed on the protective coating. An induction coil is disposed on the atmosphereic side of the envelope and formed from a plurality of windings (turns) that are parallel to each other and lie in the planes that are parallel to the tube axis. This results in the generation in the envelope of the axially uniform plasman and, hence, UV and visible radition.
An object of the present invention is to design an efficient ferrite-free electrodeless fluorescent lamp operated in a wide range of frequencies, from 100 KHz to 200 MHz and wide range of power, from 20 W to 2,000 W.
Another object of the present invention is to design an induction coil that consumes an insignificant amount of RF power in KHz and MHz range, so the efficiency of the lamp is the same or comparable to those of lamps described in U.S. Pat. No. 5,834,905 and in U.S. patent application Ser. No. 09/256,137.
Yet another object of the present invention is to locate the coil as to provide its efficient coupling with the lamp plasma.
Again another object of the present invention is to generate axially uniform plasma that generates axially uniform visible light.
A further object of the present invention is to design a lamp that is easy to manufacture and of low cost.
The many other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading the following specifications when taken in conjunction with the drawings and claims.